Polymeric materials, such as photoresist, are widely used in the semiconductor industry to produce masks of all types. In mask making the photoresist is overlaid on a surface in which the desired image is to be formed, exposed to the desired image and developed so that the image formed in the photoresist can be replicated in the underlying surface.
Presently available X-ray lithography apparatus does not have an ability to manipulate the X-rays used. Therefore, reduced mask images, as used in present day optical steppers, cannot be used. Instead each X-ray mask must be fabricated full size which, accordingly, requires particularly tight specifications for image placement, image size, and defect control. Such tight specifications cannot be obtained by optical exposure of the photoresist. To achieve the extremely fine details in the resulting replicated image on the underlying surface, necessary to X-ray masks, the photoresist is exposed by electron beams (E-beams).
When using an E-beam apparatus to fabricate masks many things can cause distortions in the images produced. Correction of such distortions is especially necessary when X-ray masks are to be produced. For example, mechanical and electrical considerations of the E-beam apparatus used can cause distortions due to effects such as stage translational errors, magnification, drift, mirror distortion, and column charging. In X-ray masks other distortions, such as stress induced errors, often occur due to the process used to deposit the gold absorber material or to remove the photoresist. Still other distortions, such as localized heating during exposure and charging of the resist itself can occur due to interaction characteristics of the electrons in the beam and the photoresist. These distortions can be pattern dependent due to the shape and layout of the pattern being created in the photoresist. Since any of these distortions appearing in the mask used to produce the final product will be replicated in the final product it is desirable that the mask be created with as few distortions as possible. It is therefore important that correction or compensation be provided for as many of the above described distortions as is possible.
Attempts to correct for some distortions in E-beam mask making have been made in the past. For example, distortions due to mechanical stage translations have been corrected on a pixel by pixel basis by first printing registration marks on each pixel of a semiconductor wafer, in which the image is to be replicated, and then comparing the actual position of such marks on the wafer with the desired position of the marks on the wafer. Such a technique is not applicable to the E-beam lithographic processed in X-ray mask making known as blind writing, where sites for such registration marks are not available on the mask. Nor does such known technique correct for process induced distortions.
Another attempt at distortion correction in E-beam tools required the incorporation of a fly-eye lens in the E-beam apparatus. However, this fly-eye lens arrangement failed to correct process induced distortions and is also not useful with blind writing.
The solutions discussed above did result in some macroscopic improvements to the quality and uniformity of images replicated in the wafer, but they corrected only the grossest errors and did not correct the remainder of the above listed distortions. Until the present invention, extremely fine replication of the image over the entire surface of a mask was not realized.
As the demand for increasingly dense semiconductor devices has increased, even the slightest variation in an image formed over the entire wafer surface creates great difficulty in meeting the demand or results in defective devices.